Liquid crystal is mainly used as a dielectric medium in liquid crystal display elements, because the optical properties of such materials can be affected by an applied voltage. Liquid crystal display elements are divided, according to operation modes, into phase change (PC), twisted nematic mode (TN), super twisted nematic mode (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), lateral electric field switching, vertical alignment (VA), polymer stabilized alignment (PSA) modes, etc.
Liquid crystal display elements used at present are mainly those liquid crystal display elements of TN type. However, they have a defect of a strong contrast and viewing angle dependence. In addition, so-called VA type liquid crystal display elements are known to have a broader viewing angle. A liquid crystal cell of a VA type liquid crystal display element contains a liquid crystal medium layer between two transparent electrodes, wherein the liquid crystal medium generally has a negative dielectric anisotropy value. Under a condition of power outage, molecules of the liquid crystal layer are aligned perpendicularly to electrode surfaces (homeotropically) or has an inclined homeotropic alignment. When a voltage is applied to the electrodes, realignment of the liquid crystal molecules parallel to the electrode surfaces occurs.
In addition, it is known that OCB type liquid crystal display elements are based on a birefringent effect and have a liquid crystal layer having a so-called “curved” alignment and a generally positive dielectric anisotropy. When a voltage is applied, realignment of liquid crystal molecules parallel to the electrode surfaces occurs. In addition, OCB type liquid crystal display elements generally comprise one or more birefringent optical retardation films to prevent a curved cell from an unexpected optical transparency under a dark condition. OCB type liquid crystal display elements, compared with TN type liquid crystal display elements, have a broader viewing angle and a shorter response time.
With regard to a PSA type liquid crystal display element, under a condition of disposing a polymerizable liquid crystal composition formed from a liquid crystal composition and a polymerizable compound between substrates, a voltage is applied between substrates as appropriate to make liquid crystal molecules aligned, and with ultraviolet irradiation under the aligned state, etc., the polymeric compound is polymerized so that the alignment state of the liquid crystal is memorized as a cured substance. At present, the PSA principle is being used in various traditional liquid crystal display elements. Therefore, for example, PSA-VA, PSA-OCB, PS-IPS/FFS- and PS-TN-display elements are known.
However, not all combinations composed of a liquid crystal composition and a polymerizable compound are suitable for PSA display elements. This is because, for example, an inclination or sufficient inclination cannot be obtained by adjustment, or because, for example, so-called “voltage holding ratio (VHR)” is insufficient in TFT display applications. Moreover, it has been found that when used in PSA display elements, there are still some defects for known liquid crystal compositions and polymeric compounds in the prior art. Therefore, not every known polymerizable compound soluble in liquid crystal composition is suitable for PSA display elements. The selected combination of a liquid crystal composition and a polymeric compound should have the best possible electrical properties, and especially, should have a VHR as high as possible. In a PSA display element, a high VHR after irradiation with a UV light is particularly necessary.
As a subject of such a liquid crystal display element, there are reliability problems such as “burned-in” produced when the same displaying continues for a long time and producibility problems caused in the manufacture process. The reliability problems are not caused by a single factor but by several complex factors, particularly caused by an inclination change of liquid crystal molecules (change in a pretilt angle).
An object of the present invention is to provide new materials suitable for a PSA type liquid crystal display element, particularly a combination of a liquid crystal composition and a polymerizable compound, which are suitable for solving the above-mentioned problems, and can overcome the above-mentioned defects when used in a PSA type liquid crystal display element. The combination of a liquid crystal composition and the polymerizable compound provided by the present invention has a larger operating temperature range, and further has an appropriate dielectric anisotropy, an appropriate optical anisotropy and a high VHR value after exposed to UV. When the combination comprising the liquid crystal composition and the polymerizable compound of the present invention is used, as compared to the prior art, it has the characteristics of a good UV resistant performance and being capable of producing a pretilt angle more quickly.